Microwave local oscillators with very low phase noise are used in various radar, communications and radiometric applications. Moderately low noise at frequency offsets of less than 1 kHz is conventionally obtained by locking a microwave oscillator to the multiplied frequency of a (e.g. 5 to 10 MHz) quartz crystal oscillator. Very low noise has been obtained by using superconducting cavities, one design using an oscillator element formed solely of superconducting material, and another using a sapphire core coated with superconducting material. Because penetration depth varies with temperature for superconductors, these oscillators require temperatures of no more than about 1.degree. to 2.degree. K. Such very low temperatures require large and expensive cooling equipment. A very low phase noise oscillator which could operate at temperatures of at least about 10.degree. K. and higher, which is obtainable by available relatively small and economical cryogenic coolers, would be of considerable value.